(1) Field of the Invention
The present invention relates to a process for regenerating electroless plating bath containing a chelating agent such as ethylenediaminetetraacetic acid (EDTA) or the like and an apparatus therefor, in particular relates to a process for regenerating electroless plating bath which comprises supplying the copper ion resultant from anode dissolution in the form of an EDTA-copper complex by virtue of the chelating agent recovered from the plating bath and an apparatus used therefor.
(2) Description of the Prior Art
Electroless plating, irrespective of whether it is used as the under-coating for electroplating or it is used by itself, is accompanied with accumulation of by-products in the plating bath resulting from the consumption of copper ion, pH modifier i.e. hydrate ion and reductant. This phenomenon should be said unavoidable because electroless plating reaction is an inversible reaction.
On the other hand, the quality of electroless copper plated film depends widely on the plating bath composition and the plating conditions. That is, with the increase of salt concentration due to the by-products in plating bath, the characteristics and quality of electroless copper plated film deteriorate and additionally the rate of plating reaction varies.
In the electroless copper plating for printed wiring board, in particular the printed wiring board prepared by semi-additive process or full-additive process, it is required that the resulting electroless plated film should possess exceedingly superior physical properties as compared with those of the electroless plated film, acting merely as only a conductive thin film for a through-hole, that is, prepared by conventional subtractive process wherein the through-hole and the circuits are mostly formed by electrolytic copper plating. That is to say, if the physical properties of electroless copper plated film are not equivalent to those of the copper film formed by electro plating, of which copper pyrophosphate plating bath and copper sulfate plating bath are typical, it will be impossible to obtain the printed wiring board equivalent in quality to that prepared by electro copper plating, and controlling of the deposition rate of electroless copper plating comes to be of great importance in the point of the control of the plated film feature. In view of this, it becomes necessary to control the electroless copper plating bath composition so as to maintain its concentration as uniform as possible and further to reduce reaction by-products as little as possible.
Controlling of bath concentration has hitherto been conducted by adding thereto separately prepared copper sulfate solution, sodium hydroxide solution and reductant such, for instance, as solid or liquid formaldehyde respectively in fixed quantities when the concentrations of components such as Cu.sup.2+, OH.sup.-, and reductant, which decrease with the progress of electroless plating reaction, in the bath are conjectured to have reached predetermined concentrations by manual or automatic analysis or from the treated mass of the substrate and times required for plating.
On the other hand, this comes to cause accumulation of sodium sulfate, sodium formate and further alcohols such as methanol, ethanol and the like. Taking account of the fact that the number of rejects of the plated products increases as these reaction by-products increase, it has been adopted by experience to disuse a part or the whole of the bath which has been used up to a certain bath life and use a fresh plating bath.
However, this way is defective in that it is not only expensive but also likely to bring about irregularity of the quality, deteriorated productivity and the like, and additionally involves problems to be settled especially when the electroless plated film of high quality as mentioned above is demanded. Further, the exchange of plating bath involves a problem on treatment of the spent bath. In more detail, at that time it becomes necessary to consider treatment for rendering the chelating agent contained in the spent bath nonpoisonous such, for instance, as COD counterplan, BOD counterplan and the like against the said chelating agent. Accordingly, it is feared that this way not only brings about increase of expenses for making the said chelating agent nonpoisonous but shall be unable to cope with the social circumstances where the disuse of spent bath per se is getting difficult from the viewpoint of environmental pollution regulation.